Unexpected hair growth can occur after tissue injury. The pathogenic mechanism for this phenomenon is unknown but is likely related to inflammatory mediators. One such mediator is high-mobility group box 1 (HMGB1), a ubiquitous nuclear protein that is released from cell nuclei after tissue damage. To elucidate the effect of HMGB1 on hair growth and understand its mechanism of action, we evaluated the effect of HMGB1 treatment on hair shaft elongation and on mRNA and protein expression in cultured human dermal papilla cells (hDPCs). HMGB1 enhanced hair shaft elongation in an ex vivo hair organ culture. In hDPCs, HMGB1 treatment significantly increased mRNA and protein expression levels of prostagladin E synthases. HMGB1 also stimulated prostaglandin E2 (PGE2) secretion from hDPCs. Finally, blocking the receptor for advanced glycation end-products, a canonical HMGB1 receptor, inhibited HMGB1-induced PGE2 production and hair shaft elongation. Our results suggest that HMGB1 promotes hair growth via PGE2 secretion from hDPCs. This mechanism can explain the paradoxical phenomenon of trauma-induced hair growth. Thus, HGMB1 can be a viable therapeutic target for the treatment of alopecia.